Reexamining the Origins of Asian Homo erectus

We have postulated that Homo erectus in China was descended from a recent immigrant from Africa. Evidence now suggests that the evolutionary transition from Homo habilis to Homo erectus happened along a very broad evolutionary front—from Africa through Eurasia. More fossil evidence from well-dated sites earlier than Longgushan Locality 1 is needed to test this hypothesis. Our hypothesis is at variance with the view among some Chinese paleoanthropologists that Homo erectus evolved in situ in the Far East and is the product of wholly regional differentiation. Fossil evidence from China from between two and three million years ago should help settle the question.

Dragon Bone Hill itself may yet provide evidence of the precursors of late Homo erectus. Hominid fossils have not yet been found in the earliest fossil-iferous layers (Layers 11 to 17) at Longgushan. Our hypothesis is that this absence of hominids at the near one-million-year mark in the cave reflects not the possibility that hominids were not in China at that point, but that the habitat and environment of deposition in the cave were not suitable for hominids at that time. Very little is known of the earliest levels at Longgushan. One pit ("the Lower Cave") has been excavated and found to have hominids down to Layer 10, and that was in 1929. We predict that hominids will not be found in the lowest levels in the same circumstances of burial as higher in the deposits because the cave was too wet and near river level. Land vertebrate fossils may be found washed in and covered by water-laid sediments, but most of the preserved fauna in these layers will likely be aquatic. Excavation of the lowest levels at Longgushan, coupled with geological and geochemi-cal investigations, could be undertaken to test this idea.

The dates that we accept for the Homo erectus fossils from Longgushan are 410,000 to 670,000 years ago, ages that make the most sense from the recent and most rigorous uranium-series, electron-spin resonance, and paleomagnetic analyses, as well as from the chronology of paleoenvi-ronmental isotope curves and loess sediment records. These dates are further back than the traditionally accepted dates for Longgushan and they cover a longer time span. The new dates are less firm than at many sites,

At two million years ago hominids were restricted to Africa. But by 1.8 million years ago Homo erectus had reached Java (a child's skull was found at Mojokerto) and by 1.7 million years ago the species is recorded as having reached western Asia at Georgia (three skulls found at Dmanisi). This map of the Old World shows the location of major hominid fossil sites discussed in this book. This is not a comprehensive list of all sites but rather is a roadmap to the key hominid sites discussed in the text. Areas in white around continental margins indicate land masses that are submerged today but that in the past, when sea levels were lower, provided land links for hominid migrations. The location of each hominid site is indicated by a small black dot. After (or before) each site name is a symbol indicating the different hominid species that can be found at the site.

♦ Homo sapiens particularly in Africa, that have good potassium-argon dates, and therefore more supporting dating data from Longgushan are needed. Models of human evolution rely on firm chronological frameworks, and valid interpretation of the uniquely important hominid fossil samples from Longgushan require that we subject our dates to continuing scrutiny. Ongoing research in geochronology should be tied to any geological and geochemi-cal research at the site.

The origins of Homo erectus now seem to lie earlier in time than the sediments at Longgushan Locality 1 can address. Other sites, perhaps on Dragon Bone Hill itself or in other parts of China, must give us knowledge of the period one to two million years ago, when we hypothesize that Homo dispersed and evolved in China. So far only three sites in southern China—Longgupo at 1.9 million years old, Yuanmo at 1.7 million years old, and Lantian at 1.1 million years old—have given us fragmentary tooth and jaw fossils documenting the earliest immigrant hominids. Other sites and sediments need to be found and explored. Exciting possibilities abound in the Nihewan Basin of northern China, which recently yielded artifacts dated at 1.36 million years ago.1 It is entirely possible that other localities on Dragon Bone Hill that are already known but still poorly investigated could be of the correct age to probe this time period and may hold some important answers. Fruits of this exploration can be expected to shed light on the model of human evolution that we propose—clinal replacement.

Clinal replacement predicts a predominantly gradual mode of evolutionary change from one species to another in human evolution. Replacement of populations occurs, but the replacements are small in scale and involve closely related populations. As we track the incursion of the first hominid species into Asia, Homo erectus ergaster, and its subsequent evolutionary change, it will become apparent whether or not this model correctly fits the facts. We and several of our colleagues argued more than 20 years ago that the fossil and geochronological evidence supported a gradual evolutionary transition from Homo habilis to Homo erectus in Africa.2 We believe that this argument still holds true and that fossil discoveries made since have confirmed our conclusions. If clinal replacement is correct, a similar gradual evolutionary transition will be discovered between Homo erectus ergaster and Homo erectus erectus in Eurasia. If an abrupt, species-level change from Homo erectus ergaster to Homo erectus erectus is found to have occurred in Eurasia, then clinal replacement as a model for Asian human evolution at this stage will be disproved.

Once hominid populations became established in Eurasia, evolutionary change then had the potential to become a two-way street. Genes could flow back into Africa from Eurasia even though the corridor of connection through the Middle East was narrow. Further paleontological investigation of sites accurately dated along the corridor of exchange and throughout

A view of the evolutionary, geographic, and temporal relationships of Homo erectus, following Philip Rightmire (2001). "S" refers to speciation events and "my" stands for million years ago. Homo erectus persisted in Asia much longer than in other parts of the world, eventually being displaced by either Homo heidelbergensis or its descendant Homo sapiens.

the entire range of hominid occupation would be essential to testing this idea. Such sites in the one-to-two-million-year-old time range that are already known include Ubediya in the Jordan Valley, Tighenif (Ternifine) in Algeria, Dmanisi in Georgia, Ceprano in Italy, and a number of archaeological sites on the Indian subcontinent that until now have yielded only stone tools. How and when these gene exchanges occurred are important questions to answer. We will continue to use morphological proxies for assessing gene exchange among ancient populations, but we can also hope for the recovery of ancient DNA, already known for Neandertals,3 to assess these deductions directly. The history of environmental change can also aid us in reconstructing scenarios and determining what actually happened in the evolutionary dynamics of Pleistocene hominid populations.